To guarantee that an extension field is processed with its associated
extension, the extension query part must escape any special characters like
'*' or '['. If the extension query contains any whitespace characters, the
extension query part must be enclosed in quotes.
Example ('_' used as separation character):

title_customExt:"Apache Lucene\?" OR content_customExt:prefix\*

Search on the default field:

_customExt:"Apache Lucene\?" OR _customExt:prefix\*

The ExtendableQueryParser itself does not implement the logic how
field and extension key are separated or ordered. All logic regarding the
extension key and field symbol parsing is located in Extensions.
Customized extension schemes should be implemented by sub-classing
Extensions.